Okay, here we go (in alphabetical order):
1. BeholderWeird, alien, and badass, these D&D-original creatures could easy take on a whole party and yawn about it afterward. I also love the fact that they are very intelligent and could very well serve as evil masterminds in a campaign. Ed Greenwood’s Forgotten Realms also seemed to give beholders credit for being movers and shakers. To this day I’ve never run one as a DM—never had a party that could survive one—but I really, really look forward to that day.
2. Carrion CrawlerThis monster works well for those times where the DM needs a mindless creature with which to threaten the party—it beats out giant ants by a long shot. I always thought giant insects were cool—too many viewing of THEM! on the late show growing up no doubt—and this one fits the bill nicely. (I was greatly disappointed to see they didn’t make the 3.5 SRD!) The Anhkheg makes a very close second place for fictional insect-type monster, but for me the carrion crawler is extremely iconic of D&D (and that’s probably what kept it out of the SRD).
3. Gelatinous CubeIconic and deadly. Despite a relatively low hit dice, these babies could sneak up on lone adventures, paralyze them, and consume them before anyone realized it happened. Also, the idea of a vaguely pinkish cube (containing a visible skull or to perhaps) gliding down a dungeon hall always reminded me of the Blob, and that’s pretty creepy. They’re relentless, and they won’t stop to bargain or parley … after all, they only want to consume you…
4. GhoulThese undead were all-around useful in a campaign. They are a good challenge for low-level parties, but dangerous enough to use with ghasts or as undead backup at later levels. The fact that elves were immune to their touch always gave the elves in a low-level party reason to shine too.
5. GnollMany of the D&D humanoids suffer from an identity complex—they’ve been draw many ways by many artists and players & DMs have defined them in their own terms, which often don’t agree. For instance, how do you picture kobolds? Little evil devil guys like in the original MM drawing? Little doglike or monkeylike guys? Or lizard-like humanoids a la 4e or Paizo’s products? You understand my point.
Gnolls however have always had a fairly consistent presentation. They seem different from the other humanoids, they prefer warmer climes and open plains, and they have the added bonus of demon worship (and Yeenoghu—which I tend to pronounce YEENY-goo instead of Yen-Oh-goo—has long been a favorite demon prince of mine).
6. Green SlimeSee Gelatinous Cube.
Very deadly! The Moathouse dungeon in T1 The Village of Hommlet had two of these … in case the first one missed. Charitable, E. Gary Gygax was not.
7. Mind FlayerAh, the dreaded illithids! They might well top the beholder in the alien-feeling role, but they serve many similar functions. What did they want? How do they interact with the Drow and other underground races? These questions were left wide-open for the enterprising DM.
The rare character that had psionics knew they wouldn’t graduate as a real power player without exchanging mind blasts with one of these underground weirdies.
8. TrollA troll could have been almost any concept initially, but the tall, green, regenerating monster of the MM was distinctive and primal. Gygax always descibed them as having nests of bones and trash, which immediately let the DM know exactly how "evolved" these brutes were. Beyond that, the picture of a troll head still trying to bite after decapitation was a vivid image indeed, crowning these guys the kings of regeneration for al time.
9. Rust MonsterI love these propeller-tailed guys. For a monster based on a plastic toy, rust monsters provide a unique threat: something that cannot hurt characters physical at all, yet they can be greatly feared.
10. StirgeAnother wonderfully variable monster. Fine for 1st-level encounters in small numbers, but fearsome indeed in large schools (or whatever a group of stirges is called— a school of ravens is actually called a murder of ravens, which might fit well here). It basically a monstrous mosquito, but this creature alwys seemed very real to me and I never had trouble imagining them.
It’s come to me that nearly all my favorites are creatures created almost whole-cloth for the game, though stirges originate from Italian myth and gnolls from Lord Dunsany and his gnoles. I’ve only listed my favorites here, not what I feel are the most practival in a campaign.
Next time: The Coolest Monsters You’ve Never Used!